Pressure sensitive adhesive tapes have been used for more than half a century for a variety of marking, holding, protecting, sealing and masking purposes. The earliest medical applications where the product was referred to as an adhesive plaster were not pressure sensitive adhesives. These were, in fact, crude mixtures of natural rubber plasticized and tackified with wood rosin derivatives and turpentine and heavily pigmented with zinc oxide. These tape-like products served their purpose but, with the advent of the truly pressure sensitive adhesives, they were replaced. The term PSA has a very precise technical definition and has been dealt with extensively in the technical literature, examples of which are Chapter 17 of Houwink and Salomon "Adhesion and Adhesives", Vol. 2, Elsivier Publishing Company, 1967, and the "Handbook of Pressure-Sensitive Technology" edited by Donatas Satas, published by van Nostrand Reinhold Company, 1982.
Fundamentally, PSA's require a delicate balance of viscous and elastic properties which result in a four-fold balance of adhesion, cohesion, stretchiness and elasticity. In essence, PSA products have sufficient cohesiveness and elasticity so that, despite their aggressive tackiness, they can be handled with the fingers and removed from smooth surfaces without leaving residue. General trade usage does not sanction the extension of the term PSA to embrace tapes and adhesives merely because they are "sticky" or because they adhere to a particular type of surface.
The prior art relating to graft copolymers is directed to modifying systems for the purpose of imparting flexibility to rigid or semi-rigid polymeric backbones. The patents of Behrens (U.S. Pat. No. 3,004,958), Gregorian (U.S. Pat. No. 3,135,717), and Milkovich (U.S. Pat. Nos. 3,786,116; 3,832,423; 3,862,267) teach how to graft side chains of polystyrene or acrylate esters onto rigid or semi-rigid backbones of polyvinyl chloride or methacrylate esters to provide flexibility and temperature and impact resistance. Block copolymers of the styrene-isoprene-styrene type have had their adhesion enhanced by grafting of methyl methacrylate as taught by Harlan (U.S. Pat. No. 4,007,311). This was done without regard for elasticity or cohesiveness. In Ambrose (U.S. Pat. No. 4,075,186), a butadiene side chain is grafted to an acrylate polymer backbone to produce a molded product which has improved impact resistance but which is tack-free. British Pat. No. 872,532 is a process patent which deals fundamentally with the preparation of graft copolymers and clearly demonstrates the advantages of grafting using different methods of grafting polystyrene onto polymethacrylate.
U.S. Pat. No. 4,554,324 discloses macromer reinforced copolymers for use as pressure-sensitive adhesives. That patent discloses that such macromer-reinforced copolymers have high shear strength. Some comparative examples are disclosed which do not have the requisite shear strength. However, the adhesives of these examples are applied to relatively non-conformable backings, i.e., polyethylene terephthalate.
The difficulty of adhering tape or other devices to the human skin has long been recognized. The irregular and complex surface of the skin presents obstacles in itself and the wide variation in the skin surface from individual to individual and from site to site compound these obstacles. Acrylic PSAs have been used for many years in medical and surgical applications. An outstanding acrylic copolymer, of the type described in U.S. Pat. Nos. 2,884,126/Re. 24,906 (Ulrich) has been employed in a porous, surgical adhesive tape, U.S. Pat. No. 3,121,021 (Copeland) with very acceptable skin adhesion performance. The desirable features of an acrylic PSA in medical applications, such as less irritation to the skin, were recognized in U.S. Pat. No. 3,321,451 (Gander), as well as the disadvantages which result from adhesion buildup when the acrylic PSA is in contact with the skin for long periods of time. According to this patent, the irritation caused by removal of the tape was overcome by including in the acrylate adhesive polymer certain amine salts which made it possible to remove the tape by washing with water which is not always feasible where high standards of sterility are being maintained.
In Gander (U.S. Pat. No. 3,475,363), the inventor has attempted to overcome the objectionable compliance failure in the acrylate PSA by employing as a crosslinking agent dimethylaminoethyl methacrylate to ensure adhesion to the skin without deleterious effects. Zang (U.S. Pat. No. 3,532,652) recognizes that acrylate PSAs suffer when used on surfaces which promote migration of oils and the like to the adhesive, thereby weakening their cohesive strength. Zang overcomes this by partially crosslinking his acrylate interpolymer with polyisocyanate. In Schonfeld (U.S. Pat. No. 4,140,115), the inventor reduces the stripping of tissue cells when his acrylate PSA is removed from the skin by blending the PSA with an unreacted polyol having a fatty acid ester pendant moiety. This adhesive has a tendency to leave objectionable residue.
Another approach to modifying PSA tape for the purpose of controlling the degree of elastic compliance and viscous flow can be found in Gobran (U.S. Pat. No. 4,260,659) which relies upon and teaches how a plurality of superimposed adhesive layers having different gradients of shear creep compliance can meet the requirement of releasable adhesion to a plastic surface such as polyethylene. In Winslow (U.S. Pat. No. 4,374,883) the shear creep compliance is achieved and the cohesive strength maintained by combining two layers of adhesive which reinforce each other. While both of these address the problem of modifying a PSA to improve compliance, neither deals with the kind of precision and control required in medical products which are adhered to the skin.
U.S. Pat. No. 4,323,557 discloses a stable chemical complex of iodine, iodide and a dermatologically acceptable room temperature tacky pressure-sensitive adhesive which is substantially free of acidic components.